The contents of the aforesaid patent U.S. Pat. No. 4,958,919 are incorporated herein by reference.
A technique was described in the aforesaid patent U.S. Pat. No. 4,958,919 for designing color-corrected optical systems using fluidal liquids as refractive elements. Examples were also disclosed of color-corrected lens systems designed according to the described technique, which have one or more liquid lens elements. One example of a color-corrected lens system disclosed in patent U.S. Pat. No. 4,958,919 was a lens triplet comprising a liquid lens element made of a Cargille liquid (identified by the code number 550206) having abnormal dispersion properties, which is confined between two glass lens elements (made of Schott BK7 and Schott F2 optical glasses, respectively) having normal dispersion properties. That particular lens triplet (which was shown in FIG. 3 of patent U.S. Pat. No. 4,958,919) is illustrated herein in FIG. 1, and has an optical prescription specified in tabular format as follows:
TABLE I ______________________________________ Surface Radius Thickness No. (inches) (inches) N.sub.D V.sub.D Material ______________________________________ 1 17.615 0.650 1.5168 64.15 BK7 2 -23.491 0.067 1.5500 20.60 550206 3 -18.072 0.350 1.6200 36.37 F2 4 83.867 58.150 Air ______________________________________
where the surfaces of the lens elements comprising the triplet are numbered consecutively from left to right in accordance with optical design convention. The "radius" listed for each surface is the radius of curvature of the surface expressed in inches. The radius of curvature of a surface is said to be positive if the center of curvature of the surface lies to the right of the surface, and negative if the center of curvature of the surface lies to the left of the surface. The "thickness" listed for a given surface is the thickness of the lens element bounded on the left by the given surface, or the thickness of the gap between the given surface and the next surface to the right thereof, where the thickness is measured in inches along the optic axis of the system.
The heading N.sub.D in the next column of Table I refers to the refractive index of the lens element bounded on the left by the indicated surface, where the value of the refractive index is given for the sodium D line, i.e., for a base wavelength of 0.5893 micron. The heading V.sub.D refers to the Abbe number for the particular lens element at the same base wavelength. The "material" listed in Table I for each surface refers to the type of optical material from which the lens element bounded on the left by the indicated surface is made.
A conventional measure of performance of an optical system is obtained by plotting the change in back focal distance as a function of wavelength over the spectral band in which the optical system is intended to operate. In FIG. 2, the change in back focal distance as a function of wavelength relative to an arbitrarily selected focal surface (here, the surface at which the focal distance is optimum for a wavelength of 0.5876 micron) is plotted for the lens triplet of FIG. 1. From the two crossings of the horizontal (i.e., wavelength) axis by the curve in FIG. 2, it is apparent that the lens triplet of FIG. 1 is achromatic (i.e., color-corrected at two wavelengths). However, from the shallowness of the curve in FIG. 2 relative to the horizontal axis, it is also apparent that the lens triplet of FIG. 1 has a significantly reduced residual chromatic aberration (i.e., secondary spectrum) in comparison with typical achromats of the prior art.